1. Field of the Invention
The present invention relates to an optical pickup apparatus that is configured to reflect a laser beam emitted from a laser light source with a reflective film of a beam splitter, thereafter, bring the laser beam to an objective lens, and apply the laser beam to an optical recording medium and, more particularly, to an optical pickup apparatus that converts a linearly polarized laser light emitted from a laser light source into circularly polarized light, to be applied to an optical recording medium.
2. Description of the Related Art
In an optical pickup apparatus that optically records and reproduces a signal, using a laser beam, to/from an optical recording medium such as an optical disc of a DVD (Digital Versatile Disc) or a CD (Compact Disc), such a type of the apparatus is known that supports, using a single optical pickup apparatus, a DVD and a CD having different recording densities from each other.
Such an optical pickup apparatus that supports a DVD and a CD includes a laser light source that emits a laser beam having a red wavelength band of 645 to 675 nm suitable for a DVD and that also emits a laser beam having an infrared wavelength band of 765 to 805 nm suitable for a CD, and switches a laser beam to be used depending on the optical disc.
The laser light source is generally configured by a semiconductor laser of a laser diode, and a multi-laser unit is more often employed that is formed by incorporating, in a single package, laser-beam emitting points emitting laser beams having different wavelengths in such a manner that those laser-beam emitting points are configured in a single laser chip or those laser-beam emitting points each are separately configured in an individual laser chip and the chips are installed in the same semiconductor base.
An optical pickup apparatus that supports a DVD and a CD uses an objective lens whose incidence plane has an annular diffraction grating formed thereon, corrects spherical aberration with respect to each of the optical discs, i.e., a DVD and a CD, by diffraction by the diffraction grating of each of the laser beams having a wavelength suitable for each of the optical discs, and thereby, secures the quality of the laser beams applied to the optical discs and allows the single objective lens to support the optical discs.
In the optical pickup apparatus supporting a DVD and a CD, the optical path may be simplified by employing the above-described multi-laser unit and single objective lens.
On the other hand, an optical pickup apparatus exists that supports the Blu-ray Disc (trademark) (hereinafter referred to as BD) standard using a laser beam having a blue-violet wavelength band of 400 to 420 nm (405 nm, for example).
Such a BD-support pickup apparatus includes optical systems for a DVD and a CD in addition to an optical system for reproduction (and recording) for a BD, since a BD apparatus supports reproduction (and recording) for a DVD and a CD.
Therefore, preferably, the BD-support optical pickup apparatus has such a configuration that the optical systems for a DVD and a CD are simplified or made compact, since a plurality of optical parts of the optical systems are incorporated in the BD-support optical pickup apparatus.
In an optical pickup apparatus, a laser beam applied to a signal layer of an optical disc is converted into a circularly polarized light, and thus, adverse effects caused by birefringence in a transparent substrate resin of the optical disc are reduced and the quality of the laser beam reflected by the optical disc is improved.
A semiconductor laser, which is a laser light source of an optical pickup apparatus used for recording/reproducing to/from an optical disc, emits a linearly polarized light due to the structure of the semiconductor laser. Therefore, the optical pickup apparatus needs a quarter-wave plate, which causes a phase difference of a quarter wavelength in the laser beam emitted from the laser light source, in order to convert the laser beam to be applied to the optical disc into a circularly polarized light.
With the quarter-wave plate, the optical pickup apparatus may cause the direction of the linear polarization of the laser beam in the outward path that brings the laser beam from the laser light source to the optical disc and the direction in the return path that brings the laser beam reflected by the optical disc to a photodetector, to differ from each other by 90 degrees. Therefore, the utilization efficiency of the laser beam may be increased using a polarizing beam splitter, so that the laser light source and the photodetector may separately be disposed each in a different optical path.
In an optical pickup apparatus that includes two laser light sources supporting a DVD and a CD which uses the laser beams having wavelengths different from each other, that brings laser beams respectively emitted from the laser light sources to a common optical path, and that allows the laser beams to support the optical discs, respectively, through a common objective lens, such a configuration is known in which the laser beams having wavelengths supporting a DVD and a CD are converted into circularly polarized light by allowing a raising reflective mirror, disposed before an objective lens in an outward path, to have a function of a quarter-wave plate, and the circularly polarized light is applied to the optical discs (see Japanese Patent Application Laid-Open Publication No. 2008-251112).
The optical pickup apparatus described in the above publication has advantages in reduction in the number of assembling processes and downsizing since the number of optical parts thereof is reduced by allowing the raising reflective mirror to have a function of a quarter-wave plate. However, a phase difference of a quarter wavelength is required to be caused in each of the laser beams having wavelengths supporting a DVD and a CD using the raising reflective mirror.
In order to cause the phase difference of a quarter wavelength by the raising reflective mirror, the optical pickup apparatus allows a laser beam incident on the raising reflective mirror to make incident on the raising reflective mirror such that the direction of the linear polarization of the incident laser beam forms a rotation angle of 45 degrees relative to the direction of the inclination of the reflecting surface, that is, a P-polarization component and an S-polarization component of the laser beam become equal to each other. Thus, the optical pickup apparatus causes the desired phase difference of a quarter wavelength in each of the laser beams having wavelengths that support a DVD and a CD using the raising reflective mirror.
However, in the above-described optical pickup apparatus, a single raising reflective mirror is required to have a function of causing a phase difference of a wide-band quarter-wave plate supporting the laser beams having wavelengths that support a DVD and a CD. Therefore, it is difficult to form a reflective film that supports both the reflectance and the phase difference function of the reflective mirror, or it is necessary that a phase difference film of the wide-band quarter-wave plate is not formed on the reflecting surface of the reflective mirror by coating but the phase difference film is formed by bonding that is disadvantageous in terms of surface accuracy and cost. Therefore, it may be considered to allow not only the reflective mirror but in another optical part to have a phase difference function without proving any dedicated wave plate.
The raising reflective mirror is used to reduce thickness of an optical pickup apparatus, and thus, the raising reflective mirror may not be included in an optical layout depending on the optical pickup apparatus. Therefore, it is desired for an optical part other than a raising reflective mirror to have the phase difference function without providing any dedicated wave plate.
When total reflection is caused as in the case of the reflective mirror, such a reflective film may be formed that reflectance thereof for S polarized light becomes substantially equal to reflectance thereof for P polarized light when a laser beam is incident thereon at a rotation angle of 45 degrees.
However, it has been turned out that as to a beam splitter, etc., including such a reflective film that a laser beam is not totally reflected thereby but a laser beam is reflected thereby and also passes therethrough, in the case where a phase difference is tried to be caused using the reflective film, when the laser beam is incident on a reflecting surface such that the direction of the linear polarization of a laser beam is set to form a rotation angle of 45 degrees relative to the direction of the inclination of a reflecting surface, the reflective film can not cause a desired phase difference.
When the reflective film does not cause total reflection, the reflectance thereof for the P-polarized light is smaller than that for the S-polarized light according to the physical law of the relationship between the incidence angle and the reflectance with respect to the S-polarized light and the P-polarized light. Therefore, according to the reflection characteristic of the reflective film formed on the beam splitter, the reflectance thereof for the P-polarized light is smaller than that for the S-polarization due to the incidence angle of the laser beam. Thus, when a laser beam is incident on the beam splitter at a rotation angle of 45 degrees relative to the direction of the inclination of the reflecting surface such that the P-polarization component and the S-polarization component of the laser beam are equal, the phase difference caused on the reflective film does not accurately become a quarter wavelength (90 degrees) due to the characteristic that the reflectance thereof for the P-polarized light and that for the S-polarization differ from each other on the reflective film. As a result, such a problem arises that the laser beam reflected by the reflective film does not become circularly polarized light.